Fe--Cr alloy sheets have been known for excellent corrosion resistance. To secure even more corrosion resistance and better heat resistance properties under far more severe conditions, various elements have been added to the alloys used in the sheets. Representative examples are Mo, Co and Al. As a result, quite excellent corrosion resistance has been achieved. Pitting corrosion potential is used as a representative index for corrosion resistance (as measured in a 3.5 vol % aqueous solution of NaCl at 30.degree. C. at a current density of 10 .mu.A/cm.sup.2). With the added elements the pitting corrosion potential of the sheets can reach 500 mV or even higher. However, all of those elements are expensive. Accordingly, in the working industry, the added amount in the sheet is limited at a sacrifice of corrosion resistance and heat resistance.
Si is less expensive than Mo, Co or Al and, in addition, improves corrosion resistance or heat resistance. Accordingly, use of Fe--Cr--Si alloys in industry is expected. As an example Japanese Laid-Open Patent Publication Sho-57/134,542 discloses ferritic stainless steel containing 0.01-5.00 wt % of Si, 0.01-5.00 wt % of Mn and 0.20-1.00 wt % of Nb and having an excellent corrosion resistance.
Unfortunately, Si has the disadvantage that, when its content is about 3.5 wt % or more, toughness of the iron alloy is radically reduced. This limits its use as a material. Moreover, processing steps such as rolling and press forming become difficult. Further, it has been said that the effect of Si for improving corrosion resistance is inferior to that of Mo, Co, Al, etc. However, when the Si content is unduly restricted, its usefulness as an anticorrosive material for an Fe--Cr--Si alloy cannot be maintained.
It has been known that, in Fe--Cr alloy systems, reduction of impurities can sometimes improve toughness and processing ability without changing the main component system. A representative example is Japanese Laid-Open Patent Publication Hei-06/033,197 in which it is mentioned that, in some products, even when Si is present, processing ability can be improved by decreasing impurities. However, when a large amount of Si is present, there is a far more significant deterioration of toughness than is common in Fe--Cr alloys and there is concern that this deterioration cannot be compensated for by any degree of improvement of toughness of a common Fe--Cr alloy as disclosed in the patent. Further, it has not yet been investigated whether corrosion resistance can be kept as high as 500 mV, expressed as pitting corrosion potential.
In Japanese Laid-Open Patent Publication Hei-03/053,025, it is disclosed that when rapid cooling is conducted after hot rolling under high stress, the toughness of an Fe--Cr--Si alloy containing 0.01-0.50 wt % of rare earth metal elements (REM) can be improved. However, such a rolling process is not common and adds cost and delay. In addition, when the properties of the conventional Fe--Cr--Si alloy are taken into consideration, it is only to be expected that the resulting corrosion resistance will have to be less than 500 mV of pitting corrosion potential.